Manufacturers of electronic circuitry and components desire thermosettable compositions to prepare laminates, such as printed circuit boards, adhesives, such as die-attach adhesives, and encapsulants having high service temperatures. "High service temperature" polyepoxide thermosets as used herein refers to three dimensionally crosslinked materials having a glass transition temperature (Tg) on the order of at least 130.degree. C., and preferably 180.degree. C. or higher.
Liquid thermosettable compositions which can be partially polymerized or cured to form an intermediate solid, semi-solid or gel thermoplastic that is stable and storable at room temperature in the semi-solid form, and easily handled and capable of being formed into desired end product configurations prior to the formation of a final cured thermoset are desired. These intermediate semi-solid materials are referred to herein as "B stage" materials; the liquid compositions that can be formed into such storage stable, but flowable thermoplastic intermediates are referred to as "B stageable" and processes for forming the intermediate are referred to as "B staging". "Polyepoxides" as used herein refer to a polymer or polymer mixture wherein at least one polymer contains at least two epoxide, ##STR1## functional groups.
Conventional polyepoxide thermosettable compositions containing curing agents, optionally with accelerators, for the curing or crosslinking reaction of the polyepoxide with the curing agent are known and are capable of forming high service temperature thermosets. When during agents that are non-reactive with polyepoxides at room temperature are employed in combination with accelerators, that do not initiate or catalyze the curing reaction at room temperature, the compositions can be stored in the liquid state for some time prior to curing. However, because of the rapid exothermic reaction of conventional curing agents with polyepoxides upon heating to elevated temperatures, it is often extremely difficult or impossible upon the application of heat to prevent the compoisition from rapidly proceeding from the liquid form directly to the final three-dimensionally crosslinked thermoset without forming a desirable B stage thermoplastic. Further, since the final service temperature of the thermoset is a direct function of the temperature employed for the curing reaction, it is often difficult or impossible to obtain a very high service temperature thermoset (Tg greater than 150.degree. C.) because of the rapid onset of curing once the minimum elevated temperatures needed to initiate curing is reached. Complicating this situation is the fact that most liquid polyepoxide thermoset compositions are formulated in organic solvents. While the majority of the organic solvent used to formulate conventional polyepoxide thermosets volatilizes upon the application of elevated temperatures, some solvent can remain trapped in the thermoset resulting in a less than a 100% solid thermoset product having undesirable physical properties for certain applications. In addition, the presence of organic solvents in the liquid polyepoxide composition may present solvent removal, flammability and potential health problems.